1. Field of the Invention
The present invention relates to a power converter apparatus and, more particularly, to a PWM power converter apparatus which can suppress low-frequency voltage distortion attributable to computing period of sampling control computation, i.e., attributable to sampling frequency.
2. Description of the Related Art
Referring to FIG. 4, a known power converter apparatus has a variable-voltage, variable-frequency power converter 1 composed of switching elements. This apparatus is adapted to covert an ordinary D.C. power into an A.C. power of desired voltage and frequency and to supply the A.C. power to a stator coil (not shown) of an induction motor 2. A rotor angular velocity detector 3 detects the angular velocity .omega..sub.r of the rotor of the motor 2. Current detectors 4.sub.U, 4.sub.V and 4.sub.W detect 3-phase currents I.sub.1U, I.sub.1V and I.sub.1W supplied from the power converter 1 to the respective phases of the stator coil of the induction motor 2. Numeral 5 denotes a 3-phase-to-2-phase converter which converts the 3-phase currents I.sub.1U, I.sub.1V and I.sub.1W derived from the current detectors 4.sub.U, 4.sub.V and 4.sub.W into values on a 2-axis rotating coordinate system (d-q coordinate system) which rotates in synchronization with the frequency .omega..sub.r of the A.C. voltage supplied to the stator coil of the induction motor 2, i.e., into stator coil currents I.sub.1d and I.sub.1q.
Numeral 6 designates a magnetic flux computing device which computes magnetic fluxes .phi..sub.2d and .phi..sub.2q which interact with the rotor (not shown) of the induction motor 2, on the basis of the stator coil currents I.sub.1d and I.sub.1q and the stator coil windings V.sub.1d and V.sub.1q on the d-q coordinate system. A 2-axis-to-3-phase converter 7 converts the 2-axis voltage commands on the d-q coordinate system, i.e., the stator coil windings V.sub.1d and V.sub.1q, into actual 3-phase instantaneous A.C. voltage commands I.sub.1U, I.sub.1V and I.sub.1W. A d-axis current controller 8 serves to control the d-axis current to the command level by, for example, performing PI (Proportional Integrating) control on the difference between the d-axis component command I.sub.1d * and the actual value I.sub.1d.
Similarly, a q-axis current controller functions to control the q-axis current to the command level by, for example, performing PI (Proportional Integrating) control on the difference between the q-axis component command I.sub.1q * and the actual value I.sub.1q. A magnetic flux controller 10 serves to control the rotor-coil interacting magnetic flux of the d-axis component .phi..sub.2d (referred to as "d-axis component magnetic flux", hereinafter) to a d-axis component magnetic flux command .phi..sub.2d * which is generated internally. Numeral 11 designates a velocity controller which controls the rotor angular velocity .omega..sub.r to an internally generated rotor angular velocity command .omega..sub.r *.
Numeral 12 designates a divider which receives outputs from the velocity controller 11 and the magnetic flux computing device 12, while 13 designates a coefficient device which receives the output from the divider 12. The divider 12 and the coefficient device 13 in cooperation compute slip frequency command .omega..sub.s *. Numeral 14 denotes a subtracting device which subtracts the d-axis stator coil current I.sub.1d from the d-axis stator coil current command I.sub.1d *. Numeral 15 denotes a subtracting device which subtracts the q-axis stator coil current I.sub.1q from the d-axis stator coil current command I.sub.1q *. Numeral 16 denotes an adding device which sums the slip frequency command .omega..sub.s * and the rotor angular velocity .omega..sub.r. Numeral 17 denotes a subtracting device which subtracts the d-axis component magnetic flux .phi..sub.2d from the d-axis component magnetic flux command .phi..sub.2d *. Numeral 18 denotes a subtracting device which subtracts the rotor angular velocity .omega..sub.r from the rotor angular velocity command .omega..sub.r *. Numeral 19 designates an integrator which integrates the output of the adder 16.
FIG. 5 is a circuit diagram showing the construction of a practical example of the power converter 1 shown in FIG. 4. In FIG. 5, numeral 21 designates a D.C. power supply. Numerals 22a to 22f indicate switching elements connected to the D.C. power supply 21 and forming arms of the three phases. Numerals 23a to 23f are diodes which are connected to the switching elements 22a to 22f, respectively, in inverted parallel relation to the switching elements. A modulating circuit 24 generates modulation signals 24a to 24f and supplies these signals to the switching elements 22a to 22f so as to turn these elements on and off, in response to the 3-phase instantaneous A.C. voltage commands V.sub.1U, V.sub.1V and V.sub.1W which have a 120.degree. phase difference and which serve as sine-wave modulated control signals. The modulation signals 24a to 24c are supplied directly to the switching elements 22a to 22c, while the modulation signals 24d to 24f are supplied to the switching elements 22d to 22f after inversion.
FIG. 6 is a circuit diagram showing the construction of a practical example of the modulating circuit 24 shown in FIG. 5. Numeral 25 denotes a carrier wave generator which generates a carrier wave (triangular wave) signal 25a, 26 denotes a comparator which compares the carrier wave signal 25a with the 3-phase instantaneous A.C. voltage commands V.sub.1U, V.sub.1V and V.sub.1W, thereby producing pulse-width-modulated (PWM) signals 26a to 26c as shown in FIG. 7. The signal 26a corresponds to the modulating signals 24a and 24d. The signal 26b corresponds to the modulating signals 24b and 24e. The signal 26c corresponds to the modulating signals 24c and 24f.
A description will now be given of the operation of the illustrated apparatus. The description will begin with the explanation of the current control. The 3-phase A.C. currents I.sub.1U, I.sub.1V and I.sub.1W, supplied from the power converter 1 to the stationary coil of the induction motor 2 are detected by the current detectors 4.sub.U, 4.sub.V and 4.sub.W, and are supplied to the 3-phase-to-2-phase converter 5. The converter 5 converts the 3-phase currents I.sub.1U, I.sub.1V and I.sub.1W into stator coil currents I.sub.1d and I.sub.1q on the 2-axis coordinate system (d-q coordinate system) which rotates in synchronization with the frequency .omega..sub.1 of the 3-phase A.C. voltage commands V.sub.1U, V.sub.1V and V.sub.1W applied to the stator coil of the induction motor 2. The conversion is conducted in accordance with the following equation (1): ##EQU1##
In the equation (1) shown above, .sub.1 indicates the phase of A.C. voltage obtained through the integrator 19, and is expressed by .theta..sub.1 =.intg..omega..sub.1 dt. The d-axis current controller 8 performs a proportional integrating operation on the difference between the d-axis current command I.sub.1d * and the stator coil current I.sub.1d of the stator coil, thus producing a d-axis voltage command V.sub.1d for the stator coil. Similarly, the q-axis current controller 9 performs a proportional integrating operation on the difference between the q-axis current command I.sub.1q * and the stator coil current I.sub.1q of the stator coil, thus producing a q-axis voltage command V.sub.1q for the stator coil. The d-axis voltage command V.sub.1d and the q-axis voltage command V.sub.1q are converted by the 2-axis-to-3-phase converter into actual 3-phase instantaneous A.C. voltage commands V.sub.1U, V.sub.1V and V.sub.1W. The conversion is conducted in accordance with the following equation. ##EQU2##
The 3-phase instantaneous A.C. voltage commands V.sub.1U, V.sub.1V and V.sub.1W thus obtained are supplied to the power converter 1, whereby desired currents are supplied to the induction motor 2.
A description will now be given of the slip frequency control. The stator coil current and the stator coil current command can be regarded as being equal to each other in each of the axes d and q to meet the conditions of I.sub.1d *=I.sub.1d and I.sub.1q =I.sub.1q *, provided that the above-described current control circuit system operates with a sufficiently high speed. In such a case,the state equation of the system of the induction motor 2, taking the stator coil currents I.sub.1d and I.sub.1q as the inputs, can be expressed by the following equations (3), (4) and (5). EQU .phi..sub.2d =.alpha..phi..sub.2d +.omega..sub.s .phi..sub.2q +.beta.I.sub.1d ( 3) EQU .phi..sub.2q =.alpha..phi..sub.2q +.omega..sub.s .phi..sub.2d +.beta.I.sub.1q ( 4) EQU .omega..sub.r =.tau.(I.sub.1q .phi..sub.2d -I.sub.1d .phi..sub.2q)(5)
In these equations, .alpha., .beta. and .gamma. are constants which are determined by the induction motor 2. The slip frequency .omega..sub.s is expressed by the following equation (6). EQU .omega..sub.s =.omega..sub.1 -.omega..sub.r ( 6)
Expressing the slip frequency also by the following equation (7), the condition of the equation (4) is transformed into the following formula (8). ##EQU3##
Since the condition .alpha.&lt;0 is met, the q-axis component magnetic flux .phi..sub.2q approaches zero as the time elapses. Thus, after a certain moment, it is possible to regard .phi..sub.2q as being 0, i.e., .phi..sub.2q =0. The command .omega..sub.s * of the slip frequency .omega..sub.s is computed in accordance with the equation (7) by the divider 12 and the coefficient device 13. The adding device 16 adds the slip frequency command .omega..sub.s * and the rotor angular velocity .omega..sub.r so as to compute the frequency .omega..sub.l of the A.C. voltage supplied to the stator coil of the induction motor 2. The integrator 19 integrates the values of the frequency .omega..sub.l to determine the A.C. voltage phase .theta..sub.l, and the 2-axis-to-3-phase converter 7 performs the conversion in accordance with the equation (7) on the basis of the A.C. voltage phase .theta..sub.l, whereby the 3-phase instantaneous A.C. voltage commands V.sub.1U, V.sub.1V and V.sub.1W are obtained. These commands V.sub.1U, V.sub.1V and V.sub.1W are applied to the power converter 1, whereby an A.C. voltage of the frequency .omega..sub.l is actually applied to the induction motor 2 by the power converter 1.
A description will now be given of the control of the magnetic fluxes. If the condition of .phi..sub.2q =0 is actually obtained in the above-described control of the slip frequency, the control of the magnetic flux is regarded as being the control of the d-axis component magnetic flux .phi..sub.2d.
On condition of .phi..sub.2q =0, the equation (3) is transformed into the following equation (9). EQU .phi..sub.2d =.alpha..phi..sub.2d +.beta.I.sub.1d ( 9)
The equation (9) shows that the d-axis component magnetic flux .phi..sub.2d can be controlled to a desired value by controlling the d-axis stator coil current I.sub.1d. The magnetic flux controller 10 conducts a proportional integrating operation on the difference between the d-axis component magnetic flux command .phi..sub.2d * and the d-axis component magnetic flux .phi..sub.2d, thereby producing the stator coil current command. I.sub.1d. The value of the d-axis component magnetic flux .phi..sub.2d is determined by the magnetic flux computing device 6.
A description will now be given of the speed control. Provided that the condition of .phi..sub.2q =0 is achieved by the described slip frequency control and that the condition of .phi..sub.2d =.phi..sub.2d * is attained by the described magnetic flux control, the aforementioned equation (5) is transformed into the following equation (10). EQU .omega..sub.r =-.gamma..phi..sub.2d *I.sub.1q ( 10)
The equation (10) shows that the rotor angular velocity .omega..sub.r can be controlled to a desired value by operating the q-axis stator coil 1.sub.1q. The speed controller 11 conducts a proportional integrating operation on the difference between the rotor angular velocity command .omega..sub.r * and the measured rotor angular velocity .omega..sub.r, thus producing the command value I.sub.1q * of the q-axis stator coil current I.sub.1q.
The known PWM converter apparatus has the described construction. In order to reduce noise produced by the load such as an induction motor, high-speed switching elements such as IGBTs are used as the switching elements. In order to attain a high switching frequency of 15 to 20, it has been necessary to set the frequency of the carrier wave (triangular wave) to the high level of 15 to 20 KHz while increasing the frequency of the sampling control computation, i.e., the sampling frequency, to the same high level as that of the carrier wave. Hitherto, however, the sampling control computation could be done only at sampling frequencies lower than the frequency of the carrier wave (triangular wave), due to, for example, the limited performance of the microprocessor which conducts the sampling control computation. Consequently, the 3-phase A.C. voltage commands V.sub.1U, V.sub.1V and V.sub.1W supplied to the power converter as sine-wave modulation control signals exhibit a stepped waveform, with the superposition of the sampling frequency which is lower than the frequency of the carrier wave (triangular wave) 25 as a result of the sampling computation, as indicated in greater scale by solid line in FIG. 3. For this reason, a periodic distortion of a low frequency is inevitably caused on the voltages after the PWM modulation, making it difficult to satisfactorily reduce the noise. Reduction in the noise is achievable to some extent by using a noise filter which eliminates noise, but the use of such filter undesirably complicates the construction of the whole apparatus.